Vasoactive intestinal peptide (VIP) is referred to as a brain-gut peptide, which is a type of biologically active peptide capable of accelerating the blood flow and lowering the blood pressure. This VIP is extracted from the porcine intestine and is comprised of 28 amino acid residues (see, for example, S. I. Said et al., Science, U.S.A., 1970, vol. 169, p. 1217). In contrast, pituitary adenylate cyclase activating polypeptide (PACAP) is a peptide being comprised of 38 amino acid residues, which was isolated from sheep hypothalamus, and the structure thereof was determined based on the bioassay system for activating adenylate cyclase in cultured pituitary cells. Two types of PACAP, i.e., PACAP38 and PACAP27, are present (see, for example, A. Miyata et al., Biochemical and biophysical research communications, U.S.A., 1989, vol. 164, p. 567). The structure of the amino acid sequence consisting of 27 amino acid residues from the N-terminus of PACAP is very similar to that of VIP. Since the amino acid sequence of VIP and that of PACAP are similar to those of secretin, glucagon, and the like, VIP and PACAP are considered to be peptides belonging to the glucagon-secretin superfamily. PACAP and VIP exhibit their biological activities via PACAP/VIP receptors. Such PACAP/VIP receptors are extensively distributed throughout the body of an organism, and therefore, PACAP/VIP is reported to have a variety of biological activities. Examples thereof include an antiasthmatic effect (JP Patent Publication (Kokai) No. 8-333276 A (1996)), a hypotensive effect (JP Patent Publication (Kokai) No. 63-179894 A (1988)), a hair-restoring effect (JP Patent Publication (Kokai) No. 1-83012 A (1989)), an effect of ameliorating male erectile dysfunction (JP Patent Publication (Kokai) No. 1-19097 A (1989)), an effect of enhancing vaginal lubrication (JP Patent Publication (Kokai) No. 1-501937 A (1989)), an effect of inhibiting the movement in the gastrointestinal tract (JP Patent Publication (Kohyo) No. 6-507415 A (1994)), an effect of ameliorating neurodegenerative diseases, hypoxia, and reduced memory skills (JP Patent Publication (Kokai) No. 7-69919 A (1995)), an effect of treating skin ulcer (JP Patent Publication (Kokai) No. 8-40926 A (1996)), acceleration of neural network construction (JP Patent Publication (Kokai) No. 2001-226284 A), and activity as an agent for ameliorating conformational diseases (Onoue S. et al., FEBS Letters, Holland, 2002, vol. 522, pp. 65-70). If such biological activities were taken into consideration, medical applications of PACAP and VIP may be very extensive. However, biologically active peptides, such as PACAP and VIP, are generally unstable, they are immediately metabolized particularly in the body of an organism, and thus, the duration of their effects is very short. Accordingly, the present inventors created a PACAP/VIP derivative possessing enzyme resistance as a novel function (JP Patent Publication (Kokai) No. 8-333276 A (1996)). They demonstrated that the PACAP/VIP derivative had excellent stability against the metabolism caused by in vivo peptidase such as trypsin (Kashimoto K. et al., Peptide Chemistry, 1996, vol. 1997, pp. 249-252), and such derivative had significant effects of prolonging drug effects by employing the effects of bronchial dilation as an indicator (Yoshihara, S. et al., Peptides, U.S.A., 1998, vol. 19, pp. 593-597; and Yoshihara, S. et al., British Journal of Pharmacology, 1997, vol. 121, pp. 1730-1734). Thus, the long-acting PACAP/VIP derivative was considered to be very useful for medical applications and it was considered to be a probable candidate as a drug targeting the aforementioned various biological and pharmacological activities. Although the biochemical stability of the PACAP/VIP derivative was confirmed, it was found that such derivative might cause serious problems in terms of pharmaceutical stability, particularly in terms of long-lasting stability in the form of a solution. Such drastic degradation gives rise to deep concern over diminished activity of PACAP/VIP, unexpected side-effects caused by impurities generated, and the like. Because of biological and pharmacological activities of PACAP/VIP, single administration of drugs comprising the same is hardly sufficient and thus, it would be easy to deduce that such drugs must be continuously administered in clinical settings over a long period of time. Therefore, generation of by-products resulting from various problems concerning stability is particularly a serious issue.